Logging devices that measure geological formations electromagnetic properties (e.g. dielectric constant and conductivity) are known, for example from U.S. Pat. No. 5,434,507. The logging device includes a transmitter and spaced receivers mounted in a pad that is urged against a borehole wall, the borehole being drilled into geological formations. Microwave electromagnetic energy is transmitted into the formations, and energy that has propagated through the formations is received at the receivers. The phase and amplitude of the wave propagating in the formations is determined from the receivers output signals. The dielectric constant and the conductivity of the formations can then be obtained from the phase and amplitude measurements.
The transmitters and receivers comprise antennas that are assimilated to magnetic dipoles. These dipoles are tangential to the pad face and are orientated in different directions. A broadside mode corresponds to the dipoles oriented orthogonally to the pad axis. An endfire mode corresponds to the dipoles oriented in alignment with the pad axis. The depth of investigation for the broadside mode is poor. The investigation depth for the endfire mode is greater than for the broadside mode, but the signal is weaker. The attenuation and phase-shift are measured between the two receivers. A simple inversion allows in case of a homogeneous formation to retrieve the dielectric constant and the conductivity. When the antennas are disposed in a two dimensional array, it is possible to obtain a permittivity ε image and a conductivity σ image of the formation surrounding the borehole.
Typically, such a logging device is unable to provide an accurate measurement of the formation properties because of its high sensitivity to the standoff of the pad relatively to the formation or the presence of a mudcake layer on the borehole wall. Even a thin film of nonconductive mud or mud filtrate constitutes an opaque layer, preventing conventional micro-resistivity imagers from measuring the formation. Oil-base mud (non-conductive mud) can be replaced with water-base mud (conductive mud), though, at considerable expense. Further, there is no guarantee that measurement will be possible. Thus, there is a need of addressing the problem of obtaining micro-resistivity images of the formation in the hereinbefore mentioned difficult environment.